Droplet Ejecting Coatings

ABSTRACT

Coating compositions are provided that eject droplets of condensed fluid from a surface. The coatings include a nanostructured coating layer and in some embodiments, also include a hydrophobic layer deposited thereon. The coating materials eject droplets from the surface in the presence of non-condensing gases such as air and may be deployed under conditions of supersaturation of the condensed fluid to be ejected. A heat exchanger design utilizing the coating is described herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.62/396,728, filed Sep. 19, 2016, and 62/397,319, filed Sep. 20, 2016,both of which are incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The invention relates to compositions that eject condensed fluids from asurface and methods of use thereof.

BACKGROUND

Water fouling is a common problem in condensation heat transferapplications. Hydrophobic coatings are currently used to promotedropwise condensation heat transfer, which has previously been shown toenhance heat transfer by as much as three-fold, relative to uncoatedsurfaces. Dropwise condensation typically involves droplets nucleatingon the surface, coalescing, and sliding down the surface due to gravity.“Jumping droplet” condensation is a relatively new phenomenon that hasbeen observed with only two hydrophobic surfaces in systems that containonly water vapor. Contrary to typical dropwise condensation, “jumpingdroplet” condensation involves droplets nucleating on the surface,growing through the further condensation of vapor and/or coalescence andthen jumping from the surface out of plane. This has been shown toenhance the heat transfer as high as 35% over typical dropwisecondensation. Non-condensing gases, such as air, have been observed toprevent the jumping droplet condensation; consequently, this phenomenonhas never been observed in the presence of non-condensing gasses such asair. Furthermore, supersaturation above 1.12 has also observed to causethe droplets to flood the surface instead of jumping, resulting inreduced heat transfer relative to a typical hydrophobic surface.(Miljkovic, N., et al., Jumping-Droplet-Enhanced Condensation onScalable Superhydrophobic Nanostrucutred Surfaces, Nano Lett.13:179-187, doi:10.1021/n1303835d (2013); Miljkovic, N., et al.,Modeling and Optimization of Superhydrophobic Condensation, J. HeatTransf-Trans. ASME 135:14, doi:10.1115/1.4024597 (2013); Milkovic, N.,et al. Condensation heat transfer on superhydrophobic surfaces, MRSBull. 38:397-406, doi:10.1557/mrs.2013.103 (2013); Aili, A., et al., inASME 2016 5th International Conference on Micro/Nanoscale Heat and MassTransfer, V001T004A001 (ASME, 2016))

One problem encountered in high efficiency HVAC applications is blowoffof condensed water vapor being entrained in the primary flow, such thatcorrosion and other undesirable outcomes occur. To combat corrosion ofheat exchanger fins, barrier coatings are often employed. In order tomaintain high water condensation efficiency (air side heat transferefficiency), coatings which modify the surface energy (to be eitherhydrophobic or hydrophilic or even biphilic (patterned areas ofhydrophobic and hydrophilic)) are applied. Hydrophilic coatings retainthe condensate on the fins in a thin layer, so as to minimize downstreamblowoff. Hydrophobic coatings are designed to rapidly shed condensatefrom the surface.

Improved surface structures that promote ejection of droplets for heattransfer and/or dehumidification, without the disadvantages of previoussystems, are needed for high efficiency applications. Improved surfacestructures that promote ejection of droplets and increase overall heattransfer can lead to heat exchanger designs with improved overallefficiency, smaller overall footprint, simpler fin designs, orcombinations of all of the above.

BRIEF SUMMARY OF THE INVENTION

Coating compositions for ejection of droplets of condensed liquid from asurface and methods of making and using such compositions are providedherein.

The coating compositions include a nanostructured layer deposited on asubstrate, and a hydrophobic coating deposited on the nanostructuredlayer. The texture and/or geometry of the nanostructured layer providesa driving force for droplet ejection.

In one aspect, coating compositions are provided that are deposited on asubstrate. The coating ejects droplets of a condensate from thesubstrate in the presence of a gas mixture that includes one or morenon-condensing gas (NCG). In some embodiments, the gas mixturecomprises, consists of, or consists essentially of air or the primarymolecular constituents of air. In some embodiments, the condensatecomprises, consists of, or consists essentially of water.

In some embodiments, the coating ejects droplets of a condensate fromthe substrate under condensing conditions, wherein the condensate isformed at supersaturation, for example, above about 1.1. In someembodiments, the coating ejects droplets of a condensate from thesubstrate under condensity conditions, wherein the droplet arithmeticmean diameter of the ejected droplets is less than about 500micrometers.

In some embodiments, the coating includes a layer of a nanostructuredcoating material. In some embodiments, the nanostructured coatingmaterial is hydrophobic. In other embodiments, the coating includes ahydrophobic functional layer deposited or layered over at least aportion of the nanostructured coating material.

In another aspect, methods of making a coating composition that ejectsdroplets of a condensate from a substrate under condensing conditionsare provided. In some embodiments, the methods include depositing ananostructured coating material on a substrate, wherein the coatingmaterial is textured such that droplets are ejected from the surface ofthe coating material. For example, the coating may be textured such thatdroplets are ejected when the cohesive forces (e.g., surface tensionforce) exceed the surface tension adhesion forces, thereby resulting ina net force vector with a component directed away from the substrate,e.g., out of the plane of the substrate. In some embodiments, thenanostructured coating material is hydrophobic. In other embodiments,the method further includes depositing or layering a hydrophobicfunctional on the surface of at least a portion of the nanostructuredcoating material.

In another aspect, a method for removal of a condensate from a surfaceis provided. In some embodiments, the method includes exposing asubstrate that includes a coated substrate (a substrate that includes acoating composition on a surface of the substrate) to a condensingsubstance, under conditions in which condensation of the substanceoccurs, wherein fluid droplets (condensed droplets) of the substance areformed on the surface and the surface ejects the droplets. In someembodiments, the coated substrate includes a nanostructured coatingmaterial, e.g., a layer of a nanostructured coating material depositedon the substrate. In some embodiments, the nanostructured coatingmaterial is hydrophobic. In other embodiments, the coating includes ahydrophobic functional layer deposited or layered over at least aportion of the nanostructured coating material.

In some embodiments, the droplets are ejected in the presence of one ormore NCG. In some embodiments, the coating ejects droplets in thepresence of a gas mixture that includes one or more NCG. In someembodiments, the gas mixture comprises, consists of, or consistsessentially of air or the primary molecular constituents of air. In someembodiments, the condensed droplets comprise, consist of, or consistessentially of water.

In some embodiments, the droplets that are ejected from the substrateare formed at supersaturation, for example, above about 1.1. In someembodiments, the droplets that are ejected from the substrate have adroplet arithmetic mean diameter less than about 500 micrometers.

In some embodiments, the coated substrate is exposed to a gas mixture,such as air, that includes particulate material, e.g., air that includesparticulates, and the method include collection of the particulatematerial, e.g., airborne particulates, in the ejected droplets, therebyremoving the particulate material, e.g., airborne particulates, from thegas mixture, e.g., air.

In another aspect, methods for heat transfer are provided, wherein thecoating compositions described herein are exposed to a condensingsubstance under conditions in which the substance condenses as dropletson the surface of the composition, and wherein the droplets are ejectedfrom the surface. The methods may include exposure of the coatingcomposition to the condensing substance in the presence of one or morenon-condensing gases (NCG(s)), such as air, component(s) of air, and/orinert gas(es). The methods may include condensation of the condensingsubstance under conditions of supersaturation, for example,supersaturation above about 1.0, about 1.1, or about 1.25. In someembodiments, the condensing substance is water, ethanol, or arefrigerant substance, or a mixture thereof.

In another aspect, heat exchangers are provided. Heat exchangers can bedesigned to utilize the coatings described herein and address theprimary problems of fouling, efficiency and corrosion. Aluminum fins aretypically used in the volume manufacture of heat exchangers owing totheir low cost and high performance. The methods described hereinaddress the application of nanostructured coatings to this constructionmaterial. Also described herein are heat exchanger designs that benefitfrom the implementation of the disclosed coatings and operation in ajumping droplet mode of condensation. Close fin spacing of at leastabout or greater than about 4 fins per inch, 8 fins per inch, 10 finsper inch, 12 fins per inch, 14 fins per inch, 16 fins per inch, 18 finsper inch, 20 fins per inch, 22 fins per inch, 24 fins per inch, 30 finsper inch, or greater permit effective heat transfer and high condensatecollection efficiency. Fin spacing as measured as the distance betweenthe fins may be as low as about 25 microns, 50 microns, 75 microns, 100microns, 200 microns, 400 microns, 800 microns, 1200 microns, 1600microns, 2000 microns, or 5000 microns. In this design, a fin coatedwith a nanostructured coating described herein is placed in closeproximity to one or more additional fins. The fins may be coated witheither hydrophobic, droplet ejection coatings as described herein, orwith hydrophilic materials such as the base coating as described herein.Small droplets that are ejected from the surface and that are too smallto impinge upon the adjacent surface are vaporized downstream, and heattransfer to these droplets causes their evaporation, with the latentheat being removed and further reducing the air temperature. Dropletsthat impinge upon adjacent plates may be collected and removed from thesystem through a condensate collection system. In certain nonlimitingembodiments, droplets are collected downstream from the droplet ejectionsurfaces.

In another aspect, heat exchangers are provided. In some embodiments,heat exchangers are provided that include a droplet ejection coatingmaterial as described herein. In some embodiments, a heat exchangerfurther includes one or more hydrophilic surface(s). Droplets may beejected from the droplet ejection coating and collected by thehydrophilic surface. In some embodiments, adjacent surfaces of the heatexchanger are coating with droplet ejection coating material and theremaining surfaces are hydrophilic. In some embodiments, condensatedroplets are formed at supersaturation, for example, above about 1.1. Insome embodiments, droplets are ejected at an arithmetic mean dropletdiameter less than about 500 micrometers. In some embodiments, thetemperature of the surface of the heat exchanger is reduced below thefreezing point of the condensate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(b) show highly magnified darkfield images of uncoatedaluminum (1(a)) and aluminum coated with a droplet ejection coating asdescribed in Example 1 (1(b)), after 12 hours under condensingconditions with water. The image appears to show significantly lesswater condensed on the coated surface of 1(b), but this is not the case.The water condensed and was rapidly ejected from the surface before thedroplets reached a measurable size as observed through a microscope.Using the method described in Example 1, the water droplets were ejectedat an average diameter of less than 500 micrometers.

FIGS. 2(a 1)-2(a 5) and 2(b 1)-2(b 5) show highly magnified darkfieldimages showing the progression of condensation on both uncoated aluminum(2(a 1)-2(a 5)) and aluminum coated with a droplet ejection coating asdescribed in Example 1 (2(b 1)-2(b 5)). As time progressed on theuncoated aluminum surface (2(b 1)->2(b 2)->2(b 3)->2(b 4)->2(b 5)),droplets nucleated and coalesced into larger droplets. These dropletsgravitationally drained by running down the surface. This process thenrepeats to remove water from a stream of NCGs. As time progresses on thesurface coated with the droplet ejection coating, the droplets nucleate,grow, and eject from the surface before they can become large enough tobe drained by gravity. This results an increased exposed area on theheat transfer surface, thus increasing overall heat transfer efficiency

FIGS. 3(a 1)-3(a 3), 3(b 1)-3(b 3), and 3(c 1)-3(c 3) show highlymagnified darkfield image progressions of sub-millimeter dropletsejecting out of plane of the condensing surface described in Example 1.Sequential frames are from videos of a vertical plate recorded at 52frames per second. The droplets ejected orthogonal to the gravity vectorand then fell downward due to gravity and moved to the right due to theairflow in a controlled wind tunnel designed to simulate the environmentinside of an air conditioning system. The camera was oriented 20 degreesto the vertical such that a narrow strip of the substrate was in thefocal plane. The degree of supersaturation varied from 1 to 2 with noloss of performance. The inset scale bars in 3(b 1) and 3(b 2) are 250μm and the inset scale bars in 3(c 1) and 3(c 2) are 100 μm. As the timeprogresses from 1 to 2 to 3, the step between each image is about 0.02seconds. The droplet ejecting in 3(b) is about 120 μm while the dropletsejecting in 3(c) are about 10 to 20 μm. As time progresses on each showndroplet ejection from 3(a 1)->3(a 2)->3(a 3), 3(b 1)->3(b 2)->3(b 3),and 3(c 1)->3(c 2)->3(c 3). The droplets can be seen on the surface in3(a 1), 3(b 1), and 3(c 1). 20 milliseconds later, the droplet can beobserved leaving the surface in 3(a 2), 3(b 2), and 3(c 2). The ejectionevent happens in a small fraction of a second as 3(a 3), 3(b 3), and 3(c3) indicate the droplet has cleared the field of view of the microscope.These ejection events happen many thousands of times per second; thesethree events are only an example of the phenomenon.

FIGS. 4a-4b show a schematic representation of a heat exchangerutilizing nanostructured coatings as described herein. 4 a shows anarrangement of fins with alternating droplet ejection and hydrophiliccoatings. 4 b shows a schematic arrangement of heat exchanger fins witha droplet ejection coated area upstream and a hydrophilic coated areadownstream from the heat exchanger fluid circulation tube in crosssection. The figures are not meant to be final design configurations,but rather to schematically convey concepts described herein.

FIGS. 5a-5d show the onset of the freezing process on the uncoated panel(left) and liquid condensate on the treated panel (right).

DETAILED DESCRIPTION

The invention provides coating compositions and methods for efficientheat transfer by ejection of condensed liquid from a surface undercondensing conditions. The compositions described herein include ananostructured coating layer on a substrate, with a hydrophobicfunctional layer deposited on the nanostructured layer. The geometry ofthe nanostructured layer provides a driving force for ejection ofdroplets from the surface. The coating compositions described hereineject droplets of condensed liquid (e.g., water) from the surface in thepresence of non-condensing gases (e.g., air or components thereof, orinert gases). In some embodiments, the condensed liquid is ejected fromthe surface at supersaturation greater than about 1, greater than about1.1, greater than about 1.2, or greater than about 1.25. In someembodiments, the droplets are ejected from the surface with anarithmetic mean of diameters less than about 2000 microns, less thanabout 1500 microns, less than about 500 microns, less than about 250microns, less than about 100 microns, less than about 75 microns, lessthan about 50 microns, less than about 25 microns, or less than about 10microns. In some embodiments, the arithmetic mean of diameters is any ofabout 2000, 1500, 500, 250, 100, 75, 50, 25, or 10 microns to about 5microns. Droplets may be ejected from the surface to enhance heattransfer, to affect ambient humidity, and/or to enhancedehumidification. In certain embodiments, the coating compositionsdescribed herein may be used in applications such as, but not limitedto, water collection and purification, condensate collection, solventand/or contaminant recovery, atomization, or humidification.

Definitions

Numeric ranges provided herein are inclusive of the numbers defining therange.

“A,” “an” and “the” include plural references unless the context clearlydictates otherwise.

A “nanostructured” coating refers to a coating composition that has afeature in at least one dimension that is less than 100 nanometers.

“Non-condensing gases” or “NCGs” refers to gasses that do not changephase at the desired condensing conditions of a vapor. Oxygen andnitrogen, for example, are NCGs when air is being dehumidified.

“Air” refers herein to a mixture of NCGs in a gaseous stream that alsoincludes condensable materials such as water vapor, and othercondensable components. For example, air may include, but is not limitedto, oxygen, nitrogen, argon and/or other inert gas(es), and more easilycondensable gases, such as water vapor and carbon dioxide. The primarymolecular constituents of air are nitrogen, oxygen, argon, carbondioxide, neon, helium, methane, and water vapor.

“Condensing conditions” refers to a condition wherein a surface iscooled below the dewpoint of a vapor.

“Supersaturation” refers to a condition when the vapor pressure of avapor is above the equilibrium vapor pressure at a given temperature andpressure. A supersaturation of 1 refers to a relative humidity of 100%and any further increase promotes condensation.

“Ejection” in reference to droplets of liquid refers to leaving asurface with a velocity that has a non-zero normal component.

“Surface tension” refers to the tension of a liquid surface caused bycohesive forces in the bulk of the liquid that pulls inward toward thebulk and tends to minimize the surface area for a given volume.

“Droplet adhesion forces” refers to the forces responsible for causing adroplet to pull outward and spread on a surface, thus preventing it fromforming a sphere. Contrarily, “cohesive forces” are those forces thatcause a droplet to pull itself inward and form a sphere, such as surfacetension.

“Refrigerant” refers to a substance or mixture used in a refrigerationcycle as the working fluid. This fluid often goes through phase changes,but need not to be effective.

“Working fluid” refers to a liquid or gas that absorbs or transmitsenergy. For example, the working fluid in an air conditioner system isthe coolant such as Freon, glycol, or water that is used to cool theprocess fluid.

“Process fluid” refers to a liquid or gas that is being treated byinteraction with the working fluid. For example, in an air conditionersystem, the process fluid is the air that is being cooled.

“Sensible heat ratio” refers to the ratio of the sensible coolingcapacity to the total cooling capacity.

Droplet Ejecting Coatings

Droplet ejecting coating materials are provided that eject condenseddroplets of liquid from the surface of a substrate. The droplet ejectingcoating material includes a nanostructured material deposited on asubstrate, and a hydrophobic material deposited on the nanostructuredmaterial. The nanostructured material includes a geometry that providesa driving force for droplet ejection from the surface. The geometry mayinclude, but is not limited to, a nanostructure that causes the dropletsto take a distorted shape upon condensation.

Droplet ejecting coating materials may include a surface that istextured such that condensed droplets are ejected when the surfacetension force exceeds the droplet adhesion forces, thereby resulting ina net force vector that has a component out of the plane of thesubstrate, for example, in applications in which the removal of theliquid from the surface or the inlet stream is advantageous. Asurprising example of such an application is the prevention of freezing,i.e., prevention of onset of frost, for example, wherein the dropletdiameter at ejection is sufficiently small, less than about 500micrometers, which prevents frost and frost adhesion on substrates ofinterest.

The coating materials disclosed herein may eject condensed fluid fromthe surface in the presence of a gas mixture, e.g., a gas mixture thatincludes one or more NCGs. For example, the coating materials may ejectfluid in the presence of air, gas components of air, or inert gases. Insome embodiments, the gas mixture includes oxygen and/or nitrogen. Insome embodiments, the gas mixture includes nitrogen, oxygen, carbondioxide, hydrogen, helium, and argon, or combinations thereof. In oneembodiments, the gas mixture consists of nitrogen, oxygen, carbondioxide, hydrogen, helium, and argon. In some embodiments, the gasmixture includes oxygen, nitrogen, carbon dioxide, and argon, orcombinations thereof. In one embodiment, the gas mixture consists ofoxygen, nitrogen, carbon dioxide, and argon. In one embodiment, the gasmixture is air.

The coating materials disclosed herein may eject condensed fluid fromthe surface at supersaturation greater than about 1.0, 1.1, 1.2, or1.25, or at supersaturation about 1.0 to about 1.1,about 1.1 to about1.25, about 1.1 to about 3.0, or about 1.1 to 5.0.

Condensed fluid droplets that may be ejected by the coating materialsdisclosed herein include, but are not limited to, water, ethanol, andrefrigerants. In some embodiments, the condensed fluid is selected fromwater, ethanol, a hydrofluorocarbon (HFC), and a hydrofluoro-olefin(HFO), or a combination thereof. In some embodiments, the condensedfluid is selected from water, ethanol, difluoromethane (HFC-32),difluoroethane (HFc-152a), pentafluoroethane (HFC-125),2,3,3,3-tetrafluoropropene (HCO-1234yf), 1,3,3,3-tetrafluoropropene(HFO1234ze), or a combination thereof. In one embodiment, the condensedfluid is water. In some embodiments, the condensed fluid is anindustrial process or working fluid.

Droplet ejecting coating materials as described herein may ejectcondensed fluid droplets from the surface having an arithmetic mean ofdiameters of less than about 2 millimeters, less than about 1millimeter, less than about 500 microns, less than about 250 microns,less than about 100 microns, less than about 75 microns, less than about50 microns, less than about 25 microns, or less than about 10 microns,or any of about 10 microns to about 25 microns, about 25 microns toabout 50 microns, about 50 microns to about 75 microns, about 75 micronsto about 100 microns, about 100 microns to about 250 microns, about 250microns to about 500 microns, about 500 microns to about 1 millimeter,about 1 millimeter to about 2 millimeters, about 10 microns to about 50microns, about 25 microns to about 100 microns, or about 100 microns toabout 500 microns.

In some embodiments, the nanostructured coating layer includesnanostructured metal, ceramic, glass, or polymer.

In some embodiments, the nanostructured coating layer includes a ceramicthat is a metal oxide. The metal oxide may be, for example, a transitionmetal oxide, tin(IV) oxide, magnesium (II) oxide (MgO), or aluminumoxide. In some embodiments, the transition metal oxide is selected fromzinc oxide, iron(II, III) oxide (Fe₃O4), iron(III) oxide (Fe₂O₃),manganese(IV) oxide (MnO₂), manganese(II, III) oxide (Mn₃O₄),manganese(III) oxide (Mn₂O₃), nickel(II) oxide (NiO), nickel(III) oxide(Ni₂O₃), zirconium(IV) oxide (ZrO₂), titanium(IV) oxide (TiO₂),chromium(III) oxide (Cr₂O₃), copper(II) oxide (CuO), cobalt(II) oxide(CoO), cobalt(III) oxide (Co₂O₃), and cobalt(II, III) oxide (Co₃O₄).

In some embodiments, the nanostructured coating layer includes a glass.In some examples, the glass includes silica or a silicate.

In some embodiments, the nanostructured coating layer includes apolymer. In some examples, the polymer is a fluoropolymer, polyethylene,or polypropylene. In some embodiments, the polymer is a fluoropolymerselected from polytetrafluoroethylene (PTFE), polyvinylidene (PVDF),polyvinylfluoride (PVF), and fluorinated ethylene propylene (FEP), or acombination thereof. In some embodiments, the polymer is a blockcopolymer, for example, but not limited to, wherein each block of thecopolymer is less than about 500 monomer units, or less than about 200monomer units. For example, the block copolymer may by a hydrophobicpolymer that includes two or more monomer units. In some embodiments,the block co-polymer may include one or more monomers, such as, but notlimited to, propylene, ethylene, tetrafluoroethylene, trifluoroethylene,vinylfluoride, hexafluoropropoylene, 1,1-difluoroethylene,1,2-difluoroethylene, and isobutylene.

In some embodiments, the hydrophobic coating layer may include one ormore hydrophobic functionality selected from alkyl, vinyl, phenyl, andfluoroalkyl. For example, the hydrophobic functionality may include, butis not limited to, alkylsilane, vinylsilane, phenylsilane, orfluoroalkylsilane. In certain nonlimiting embodiments, the hydrophobicfunctionality is hexamethyldisilazine, sodium methylsiliconate,potassium methylsiliconate, dimethiconol, perfluorooctyltriethoxysilane,perfluorodecyltriethoxysilane, perfluorooctyltrimethoxysilane,perfluorodecyltrimethoxysilane, octadecyltriethoxysilane,methyltrimethoxysilane, methyltriethoxysilane, isobutyltrimethoxysilane,isobutyltriethoxysilane, or phenyltriethoxysilane. In some embodimentsthe hydrophobic coating refers to a functionalized perfluoropolyethersuch as perfluoropolyether silane, perfluoropolyether phosphonic acid,or perfluoropolyether phosphonate. In some embodiments, the hydrophobiccoating refers to a coating that when added to a smooth substrate,imparts a contact angle greater than or equal to 90 degrees.

Methods of Making Droplet Ejection Coatings

Methods of making coatings that eject droplets of condensed liquid froma substrate, e.g., under condensing conditions, are provided. In someembodiments, the methods include: (a) depositing a nanostructuredcoating layer on a substrate; and (b) depositing a hydrophobicfunctional layer, i.e., a hydrophobic material that includes one or morehydrophobic functional groups, on the surface of the nanostructuredmaterial. In other embodiments, the methods include: depositing a layerof nanostructured coating material on a substrate, wherein thenanostructured coating material is hydrophic or includes one or morehydrophobic functional groups on its surface.

The nanostructured layer may be deposited on the substrate by anysuitable means, including but not limited to, sol gel processing,chemical bath deposition, dip coating, spray coating, physical vapordeposition, or chemical vapor deposition. In one embodiment, thenanostructured coating is a metal oxide that is deposited by, forexample, sol gel processing, chemical bath deposition, or dip coating.The hydrophobic functional layer may be deposited onto thenanostructured layer by any suitable means, including but not limitedto, vapor deposition or dip coating.

Nonlimiting examples of nanostructured and hydrophobic coating materialsare described above. The substrate may include a metal, metal alloy,glass, polymer or ceramic material.

In some embodiments, the substrate is pretreated prior to deposition ofa nanostructure coating layer as described herein, to remove debris orsubstance(s) on the surface and/or to smooth the surface (i.e., toaccess the substrate to promote adhesion and to prevent defects), withone or more treatment(s) selected from cleaning, degreasing, rinsing,etching, desmutting, oxidizing, removing previous treatments,roughening, planarizing, steam cleaning, thermal oxidation, andsmoothing.

Methods of Using Droplet Ejecting Coatings

Methods of using coatings as described herein for removal of fluid fromsurface, for heat removal, for removal of airborne particulate andcontaminants, for dehumidification, and/or for the prevention offrosting are also provided. The methods include exposing a substratewith a droplet ejecting coating thereon, as described herein, to acondensing substance, under conditions in which condensation of thesubstance occurs, wherein fluid droplets of the substance are formed onthe surface, and wherein the surface ejects the droplets. In someembodiments, the methods include (a) depositing a nanostructured coatinglayer on a substrate; and (b) depositing a hydrophobic functional layeron the surface of the nanostructured material, and (c) exposing thecoated substrate to a condensing substance, under conditions in whichcondensation of the substance occurs, wherein fluid droplets of thesubstance are formed on the surface, and wherein the surface ejects thedroplets.

In some embodiments, exposing the coated substrate to the condensingsubstance occurs in the presence of one or more gases, e.g., comprisingor consisting of one or more NCGs. In one example, the coated substrateis exposed to the condensing substance in the presence of air. In someembodiments, the coated substrate is exposed to the condensing substancein the presence of a gas mixture that includes one or more of nitrogen,oxygen, carbon dioxide, hydrogen, helium, and argon, or a combinationthereof. In some embodiments, the quantity of the gas mixture to whichthe coated substrate is exposed may be, for example, about 1 ppb toabout 10 ppm, greater than about 5 ppm, greater than about 1%, orgreater than about 20%, or in certain embodiments up to 100%, forexample, to separate water vapor from an air stream, or to separatecondensable substance(s) from a gas stream.

In some embodiments, exposing the coated substrate to the condensingsubstance occurs at supersaturation of the substance. In one example,the supersaturation is greater than about 1.1. In some embodiments, thesupersaturation is greater than any of about 1.0, 1.1, 1.2, or 1.25, oris about 1.0 to about 1.1, about 1.1 to about 1.25, about 1.1 to about3.0, or about 1.1 to about 5.0

In some embodiments, the substrate may be used in a working environmentwith no external electric field or bias.

The condensing substance may be any substance that condenses on asurface and that is desired to be removed from the surface, for example,for purposes of reducing the temperature at which onset of condensationoccurs or to increase the amount of condensation that occurs at a giventemperature. For example, the condensing substance may be water,ethanol, or a refrigerant. In one embodiment, the condensing substanceis water vapor. In some embodiments, the droplets that are ejected maychange the properties of an industrial process or working fluid.

The condensing substance may be any substance that condenses or adhereson a surface and that is desired to be removed from the surface, forexample, for the purposes preventing the formation of adhered frost orice on the surface. For example, the condensing substance may be water,wherein the surface temperature is below the freezing point of water,and the water is removed from the surface prior to the formation of thesolid phase (frost or ice). In another embodiment, the condensingsubstance may be water, wherein the surface temperature is below thefreezing point of water, and the liquid droplet ejects partially from awetted to a dewetted state, which depresses the apparent freezing pointof the liquid.

In some embodiments, the coatings described herein enhance theefficiency of a distillation system, e.g., a vapor distillation device,by increasing the condensation temperature, thereby reducing the energyrequirements for the distillation system.

In some embodiments, the coatings described herein reduce the energyrequirements for condensation in process equipment, such as, forexample, a condenser, e.g., a distillation reflux condenser.

In some embodiments, the coatings described herein increase thecondensation performance in process equipment, such as, for example, avent gas stream, e.g., a knock-out pot.

In some embodiments, an external force, e.g., air flow, vibration,and/or droplet coalescence, provides excess energy in addition to thesurface tension, to effect droplet ejection.

In some embodiments, ejected condensate droplets may be directed andcollected. For example, an external collection unit or part, e.g., anadjacent external collection unit or part, may be used to collectcondensed, ejected liquid. Examples of such a collection unit or partinclude, but are not limited to, a heat exchanger (e.g., heat exchangerfin), a screen, a filter, a mist collector, and a condensate collectionpan.

In some embodiments, droplets are ejected into a cooled gas mixturestream, e.g., a gas mixture stream that includes one or more NCG, e.g.,air, and revaporized, thus decreasing the temperature of a working orindustrial process fluid. For example, droplets ejected into the cooledgas mixture stream may vary the sensible heat ratio of the cooledworking or industrial process fluid. In one embodiment, the condensingsubstance is water and the working or process fluid is air, and therelative humidity is increased and the temperature is decreased, thusincreasing the sensible heat capacity of the heat exchanger.

In some embodiments, ejected droplets will interact with airborneparticulate and collect this material to the droplet. Several dropletscan be collected in a variety of ways and the collected particulate canbe removed from the system.

In some embodiments, ejected droplets will interact with airbornecompounds. Airborne compounds which are soluble in the ejected dropletmaterial will be solubilized and collected by the ejected droplets.Airborne compounds which are capable of suspension in the ejecteddroplet material will be suspended and collected by the ejecteddroplets. Several droplets can be collected in a variety of ways and thecollected compounds can be removed from the system. For example, ejectedwater droplets may collect additional droplets of airborne substances orfacilitate heterogeneous condensation.

Heat Exchanger Designed for Use with Droplet Ejecting Coating

A system. that includes alternating fins of hydrophilic and hydrophobic(e.g., droplet ejection) coatings and methods of making same areprovided. Hydrophilic coating areas can be generated in conjunction withthe hydrophobic (e.g., droplet ejection) coatings described herein bysubmerging and processing alternate fins separately and reassembling, orgenerating adjacent areas of droplet ejection and hydrophilic propertiesby partially submerging the part to be coated after the initialdeposition of the nanostructured coating. A previously assembled unitmay be partially coated with hydrophobic (e.g., droplet ejection)coating upstream and remain hydrophilic downstream, via partialsubmersion during the coating process. An assembled unit may be coatedwith a hydrophilic material, and subsequently partially coated with ahydrophobic coating.

A structure in which alternating fins are coated with hydrophobic (e.g.,droplet ejection) and hydrophilic material is described. Condensateformed on hydrophilic coated fins wicks water, for return to acondensate drip pan. Condensate formed on hydrophobic (e.g., dropletejection) material coated fins ejects condensate, to be collected onadjacent fins or collection apparatus for return to a condensate drippan.

A heat exchanger or other system in which some areas (e.g., upstreamareas) are coated with hydrophobic (e.g., droplet ejection) materialsand other areas (e.g., downstream areas) are hydrophilic is described.In some embodiments, condensate formed on droplet ejection coated areasand ejected may be collected by downstream hydrophilic materials. Insome embodiments, any part of the heat exchanger from which droplets areto be ejected may be coated with droplet ejection material as describedherein, and remaining parts that will collect the condensate may be ahydrophilic material. For example, the chassis, drip pan, etc. may becoated with the droplet ejection) material, in addition to, oralternatively to, fins. In some embodiments, adjacent surfaces of theheat exchanger are coated with droplet ejection (e.g., hydrophobic)coating and the remaining surfaces are hydrophilic. In some embodimentsthe hydrophilic material is a coating, and in other embodiments thesurface is inherently hydrophilic or is rendered hydrophilic by asurface treatment.

A system that includes fins that include droplet ejection coatings andmethods of making the same are provided. A structure in which fins arecoated with droplet ejection material is described. Condensate formed ondroplet ejection material coated fins ejects condensate, to be collectedfor removal. Condensate formed on droplet ejection coated areas andejected may be collected by downstream hydrophilic materials. In someembodiments, any part of the system (e.g, heat exchanger) from whichdroplets are to be ejected may be coated with droplet ejection materialas described herein, and remaining parts that will collect thecondensate may be a hydrophilic material. For example, the chassis, drippan, etc. may be coated with the droplet ejection material, in additionto, or alternatively to, fins. In some embodiments, the hydrophilicmaterial is a coating, and in other embodiments the surface isinherently hydrophilic or is rendered hydrophilic by a surfacetreatment. In other embodiments, ejected condensate coalesces naturally(e.g. droplet-droplet) or actively (e.g. mesh screen coalescer) and isremoved by gravity or other accelerations of the gas mixture stream(e.g., cyclone or bend in stream).

Heat Exchanger Designed for Use with Droplet Ejecting Coating forAntifrost

A heat exchanger or other system that reduces or eliminates frostformation, and methods of making the same, are provided. Such a systemthat includes a droplet ejection coating material as described herein,and frost formation is reduced or eliminated in comparison with anidentical system that does not include the droplet ejection coatingmaterial. Condensate formed on surfaces that contain the dropletejection coating is removed via the droplet ejection mechanism describedherein. A structure in environmental surface temperatures and conditionsin which condensate retained on the surface would eventually form asolid frost or ice ejects droplets of the condensate via the dropletejection mechanism as described herein, thereby delaying or preventingaltogether the formation of solid frost or ice. The formation of frostand ice is detrimental to the efficiency of such devices and additionalequipment, and methods and materials described herein are used to removeor prevent formation of the frost and/or ice. Structures that includedroplet ejection coatings also reduce the time and/or energy required toremove frost and/or ice. Systems may be designed using the dropletejection coating materials described herein to minimize energy use fordefrosting and/or deicing. For example, the defrosting cycle may beoptimized for energy minimization. For example, the defrosting systemand hardware may be modified for energy minimization (e.g., lower power)and/or removed altogether.

The following examples are intended to illustrate, but not limit, theinvention.

EXAMPLES Example 1

Droplet ejecting coatings were created by submerging a cleaned substratein an aqueous equimolar solution of an alkaline earth metal ortransition metal salt and hexamine with concentrations ranging from 10mM to 1 M for a time ranging from 5 minutes to 3 hours. For example, analuminum substrate was used with zinc nitrate as the transition metalsalt at a concentration of 50 mM, and a temperature of 90° C. forduration of 2 hours. The substrate was then removed from the solution,rinsed, and air dried. The coated substrate was then baked at atemperature ranging from 100° C. to 600° C. for a duration ranging from5 minutes to 48 hours. For example, the coating was baked at atemperature of 550° C. for a duration of 24 hours. The surface was thensubmerged in a dilute acid rinse solution between the pH of 1 and 7 fora duration ranging from 15 seconds to 24 hours. For example, thesubstrate was submerged in an acid solution at a pH of 3 for 24 hours.The substrate was then placed in a solution containing the hydrophobicfunctional molecule at a concentration ranging from 0.05% to 2% for aduration ranging from 2 seconds to 48 hours. For example, a 1% solutionof perfluorodecyltriethoxysilane was created in azeotropic ethanol. Afew drops of acetic acid were added to catalyze the reaction. Thesubstrate was then submerged in the solution for 24 hours, rinsed, andthen annealed at 120° C. for an hour. The substrate was then cooled in astream of supersaturated air to eject droplets from the surface.

Results are shown in FIGS. 1(a)-1(b), FIGS. 2(a 1)-2(a 5) and 2(b 1)-2(b5), and FIGS. 3(a 1)-3(a 3), 3(b 1)-3(b 3), and 3(c 1)-3(c 3).

Example 2

A droplet ejecting coating is created by dipping a cleaned substrate ina solution of a block copolymer consisting of two or more monomers withat least one of the monomers capable of being formed into a hydrophobicpolymer. For example, a 1% solution ofpoly(tetrafluoroethylene-block-ethylene) is created using a solvent orsolvent mixture. The polymer block length can range from 50 to 500monomer units, such as a length of 500 monomer units. For example, thesolvent is a Vertrel/trichlorobenzene mixture in a 50/50 ratio. Thesubstrate is then removed and allowed to dry. The substrate is thencooled in a stream of supersaturated air to eject water droplets fromthe surface.

Example 3

A droplet ejecting coating is created by dipping a cleaned substratewith a roughened texture in a solution of a block copolymer consistingof two or more monomers with at least one of the monomers capable ofbeing formed into a hydrophobic polymer. For example, the substrate isdipped into a melt of block copolymer of fluorinated ethylene propylene(FEP) wherein the polymer block lengths can range from 50 to 500 monomerunits, such as a length of 500 monomer units. No solvent is required asthe coating is formed from the melt. The substrate is then cooled in astream of the super saturated air to eject water droplets from thesurface.

Example 4

A 200 aluminum mesh of 5056 aluminum alloy was coated conformally usingthe process as described in Example 1, leaving the open area largelyunchanged. The mesh had the following characteristics:

Mesh Size 200 × 200 Opening Size 0.0029″ (75 microns) Open Area 34% WireDiameter 0.0021″ (53 microns)

Example 5

A 1 inch (in.)×2 in. 3003 aluminum alloy panel was coated conformallyusing the process as described in Example 1. A similar 1 in.×2 in. 3003aluminum alloy panel was untreated. The panels were mounted to anelectrically cooled module in a side by side fashion. The temperature ofthe cooled module was reduced to the point where condensation wasobserved and droplet ejection condensation was noted on the treatedpanel. Condensation was noted on the untreated panel. The cooled moduletemperature was further reduced to approximately 5° F. of subcooling,and freezing of the droplets was observed on the untreated panel,whereas freezing did not occur on the treated panel.

FIGS. 5a-5d show the onset of the freezing process on the uncoated paneland liquid condensate on the treated panel.

Although the foregoing invention has been described in some detail byway of illustration and examples for purposes of clarity ofunderstanding, it will be apparent to those skilled in the art thatcertain changes and modifications may be practiced without departingfrom the spirit and scope of the invention. Therefore, the descriptionshould not be construed as limiting the scope of the invention.

All publications, patents, and patent applications cited herein arehereby incorporated by reference in their entireties for all purposesand to the same extent as if each individual publication, patent, orpatent application were specifically and individually indicated to be soincorporated by reference.

1.-27. (canceled)
 28. A heat exchanger that comprises coated surfaces,wherein the coated surfaces comprise a nanostructured coating materialdeposited on regions of a metal surface of the heat exchanger and ahydrophobic functional layer coated over the nanostructured coatingmaterial, and wherein when the heat exchanger is in operation and isexposed to a process fluid under conditions in which condensation of atleast a portion of condensable components in the process fluid occurs,droplets of a condensate from the condensable components are formed onthe coated surfaces and the coated surfaces eject the droplets.
 29. Aheat exchanger according to claim 28, wherein the nanostructured coatingmaterial comprises a metal oxide that is deposited onto the metalsurface of the heat exchanger without the introduction of an electriccurrent.
 30. A heat exchanger according to claim 28, wherein thenanostructured coating material and the metal surface of the heatexchanger comprise different metals.
 31. A heat exchanger according toclaim 28, further comprising one or more hydrophilic surface(s), whereinat least one hydrophilic surface is adjacent to at least one coatedsurface, wherein the adjacent coated surface is hydrophobic, and whereinwhen the heat exchanger is in operation, at least a portion of thedroplets that are formed on the hydrophobic coated surface transfer toand traverse the hydrophilic surface as a liquid condensate.
 32. A heatexchanger according to claim 31, wherein the liquid condensate iscollected by a downstream condensate collector.
 33. A heat exchangeraccording to claim 31, wherein the hydrophilic surface(s) comprise ananostructured material.
 34. A heat exchanger according to claim 28,wherein the droplets are ejected in the presence of one or morenon-condensing gas.
 35. A heat exchanger according to claim 28, whereinthe droplets are ejected at supersaturation of the condensate aboveabout 1.1.
 36. A heat exchanger according to claim 28, wherein thedroplets are ejected at an arithmetic mean droplet diameter less thanabout 500 micrometers.
 37. A heat exchanger according to claim 28,wherein when the heat exchanger is in operation, frost formation on theheat exchanger is reduced or eliminated in comparison with a heatexchanger that does not comprise the coated surfaces, when operatingunder similar conditions.
 38. A heat exchanger according to claim 29,wherein the metal oxide of the nanostructured coating material comprisesa transition metal oxide, tin (IV) oxide, magnesium (II) oxide, aluminumoxide, or an earth metal oxide.
 39. A heat exchanger according to claim29, wherein the nanostructured coating material comprises zinc oxide,magnesium (II) oxide (MgO), iron (II, III) oxide (Fe₃O₄), iron (III)oxide (Fe₂O₃), manganese (IV) oxide (MnO₂), manganese (II, III) oxide(Mn₃O₄), manganese (III) oxide (Mn₂O₃), nickel (II) oxide (NiO), nickel(III) oxide (Ni₂O₃) chromium (III) oxide (Cr₂O₃), copper (II) oxide(CuO), cobalt (II) oxide (CoO), cobalt (III) oxide (Co₂O₃), or cobalt(II, III) oxide (Co₃O₄).
 40. A heat exchanger according to claim 28,wherein the hydrophobic functional layer comprises one or morehydrophobic functionality selected from alkyl, vinyl, phenyl, andfluoroalkyl.
 41. A heat exchanger according to claim 40, wherein thehydrophobic functional layer comprises alkylsilane, vinylsilane,phenylsilane, or fluoroalkylsilane.
 42. A heat exchanger according toclaim 28, wherein the nanostructured coating material is immobilized onthe metal surface of the heat exchanger by heat treatment at atemperature from 100° C. to 600° C.
 43. A heat exchanger according toclaim 28, wherein the nanostructured coating material is deposited ontothe metal surface of the heat exchanger in a solution comprising: analkaline earth metal or a transition metal salt; and hexamine.
 44. Aheat exchanger according to claim 28, wherein the metal surface of theheat exchanger comprises aluminum and the nanostructured coatingmaterial comprises zinc and/or manganese.
 45. A heat exchanger accordingto claim 28, wherein retention of the condensate on the heat exchangersurface is prevented when the temperature of the surface of the heatexchanger is below the dew point of the condensable components of theprocess fluid.
 46. A heat exchanger according to claim 28, whereinformation of adhered frost or ice on the heat exchange surface isprevented when the temperature of the surface of the heat exchanger isbelow the freezing point of the condensable components of the processfluid.
 47. A heat exchange system, comprising the heat exchanger ofclaim
 28. 48. A heat exchange system according to claim 47, wherein thenanostructured coating material comprises a metal oxide that isdeposited onto the metal surface of the heat exchanger without theintroduction of an electric current.
 49. A heat exchange systemaccording to claim 47, wherein the nanostructured coating material andthe metal surface of the heat exchanger comprise different metals.
 50. Aheat exchange system according to claim 47, wherein the system comprisesa component that is downstream of the heat exchanger, and wherein whenthe system is in operation, ejected droplets are collected on a surfaceof the component and are removed from the process fluid stream.
 51. Aheat exchange system according to claim 50, wherein the component is amesh screen coalescer, a cyclone, a filter, a mist collector, or a bendin the process fluid stream.
 52. A heat exchange system according toclaim 50, wherein at least a portion of the component is coated with ahydrophilic material.
 53. A heat exchange system according to claim 52,wherein the hydrophilic material comprises a nanostructured materialthat enhances transfer of fluid condensate to a collector, and whereinthe condensate is removed from the system when the system is inoperation.
 54. A heat exchange system according to claim 53, wherein thenanostructured coating material is a ceramic material that requires lessenergy to defrost than an equivalent component that does not comprisethe nanostructured material.
 55. A heat exchange system according toclaim 47, wherein the system comprises a closed loop thermodynamic cycle56. A heat exchange system according to claim 47, wherein the systemcomprises a refrigeration cycle.
 57. A heat exchange system according toclaim 47, wherein the system comprises a heat pump.